The present invention relates to a method for determining the content of SO.sub.2 and more particularly, the invention relates to a method for causing SO.sub.2 to oxidize and for measuring chemi-luminescence as produced during and pursuant to the chemical reaction so as to determine the SO.sub.2 content in a gaseous atmosphere, particularly air.
Chemi-luminescence has been observed during reactions involving nitrogen oxide, ozone and other gaseous impurities and pollutants of the air. Accordingly, this phenomenon has been used in rather powerful methods for analytic determination of these pollutants. Usually the test gas is mixed with a reaction medium or reagent, either under normal or under reduced pressure. For example, upon determining the NO content, one will use ozone, or if one wishes to test the ozone content one will use ethylene. As the components so added react the resulting chemi-luminescence is photoelectrically detected; one uses here photomultipliers, secondary electron emission type multipliers etc. The resulting electrical signal is usually amplified and indicated or recorded. The measured value is indicative of the concentration of the substance sought to be detected.
Another known method provides for intimate contact between the test gas and a thin layer of liquid which contains a substance that will react with the substance to be detected under development of luminescence. The U.S. Pat. No. 3,712,793 discloses the detection of SO.sub.2 by means of light flashes resulting on contact of that substance with a thin layer of a water solution of ozone.
Another method has become known through one of us, and was published in 1965 in Photochem. Photobiol. Vol. 4, page 1199. In accordance with that method chemi-luminescence is produced in a water solution of SO.sub.2, in that sulfite and bisulfite ions thereof react with oxygen in the presence of a catalyst such as iron(III)-hydroxide. Again, the intensity of the luminescence depends on the content of SO.sub.2 in the solution. SO.sub.2 can be analytically determined in that manner but a gaseous oxidizer such as oxygen itself or ozone will react with a liquid very little or practically not at all, because the concentration of such an oxidizer cannot really be maintained at a sufficiently constant level. On the other hand, if one wants to determine quantitatively a yet unknown substance under utilization of chemi-luminescence, it is imperative to have the substance, with which the unknown substance is supposed to react under development of chemi-luminescence, available at a constant concentration, because the reaction and the resulting phenomenon depends indeed on that concentration.
Utilization of nonspecific oxidizers such as oxygen or ozone has the additional drawback that the test gas (e.g. air) may contain various substances which react also under production of luminescence. For example, ozone when reacting with NO produces chemi-luminescence, even when in the gaseous state, and that luminescence is superimposed upon the one developed and used for detecting the also present SO.sub.2.